Genomic characteristics of Salmonella enterica serovar Blockley.

Non-typhoidal Salmonella (NTS) is a significant cause of foodborne illness worldwide, with increasing antimicrobial resistance posing a public health concern. Salmonella enterica serovar Blockley (S. Blockley) is relatively uncommon, and its antimicrobial resistance profile and population structure have been understudied. This study presents a comprehensive genomic analysis of 264 S. Blockley isolates from diverse geographical regions to elucidate antimicrobial resistance patterns and population structure. Bayesian analysis classified these genomes into 10 distinct groups (BAPS A to BAPS J), further categorized into two lineages, R and S. Lineage R comprised six BAPS clusters (BAPSs A-F), predominantly found in Asia and Africa, all of which harbored the azithromycin resistance gene mph(A) and other resistance determinants. In contrast, lineage S, lacking mph(A), comprised the remaining four BAPS clusters, which were primarily found in Europe and the Americas. Several types of mutations in gyrA were found in lineage R, which were specific to BAPS clusters. These BAPS clusters exhibited distinct geographic distributions, with BAPS B, BAPS D, and BAPS E unique to China, Taiwan, and Japan, respectively, while BAPS H and BAPS I were predominantly found in the United States. Temporal phylogenetic analysis suggested that lineage R diverged in the 1980s, with notable microevolutionary changes. The presence of a genomic island with mph(A), aph(3')-Ia, aph(3")-Ib, aph(6)-Id, and tet(A) in lineage R underscores the public health threat, highlighting a need for continuous surveillance.IMPORTANCEAntimicrobial resistance in Salmonella is a global public health concern. In this study, we focused on serovar Blockley, and a whole-genome analysis revealed its global population structure. The results revealed the existence of azithromycin-resistant strains, which were characterized both phylogenetically and geographically. The resistance genes were transmitted via genomic islands, and their micro-scale evolution was also revealed. Our findings are the first to reveal the dissemination of antimicrobial resistance genes, including azithromycin, in serovar Blockley, and provide valuable insights into understanding the spread of antimicrobial resistance.